JP 2003-075108 A discloses a rotation angle sensor comprising a rotating disc-shaped magnet, and pluralities of magnetic sensors arranged near the periphery of the magnet on a plane substantially perpendicular to the rotation centerline of the magnet for detecting a magnetic field to supply an output corresponding to the rotation angle of the magnet. It shows in FIG. 4 a rotation angle sensor comprising a two-pole, disc-shaped, magnet supported by a rotation shaft, and magnetic sensors A and B disposed around the periphery of the magnet. The magnetic sensors A and B are Hall elements for detecting the intensity of a magnetic field to calculate the rotation angle. The magnetic sensors A and B are arranged such that a mechanical angle between a straight line passing the center O of the disc-shaped magnet and the center of the magnetic sensor A and a straight line passing the center O and the center of the magnetic sensor B is about 90°. The mechanical angle is 360° per one round of a disc. These magnetic sensors A and B are located at positions deviated from the magnet in the rotation axis direction. JP 2003-075108 A also shows in FIG. 6 a rotation angle sensor comprising magnetic sensors A and B inclined from the rotation axis.
However, because the output of a Hall element drastically decreases when the distance between the Hall element and a rotating magnet surface (spacing) increases, the Hall element is so vulnerable to spacing variation that it cannot easily detect the rotation angle with high accuracy. Further, because ambient temperature change leads to large variation of output due to the change of magnet characteristics (change of magnetic flux), it cannot detect the rotation angle stably.
The magnetic sensors A and B (Hall elements) are not magnetic sensors detecting the change of a magnetic flux direction. Namely, when a Hall element is inclined from the magnetic flux direction, it detects only a magnetic flux component perpendicular to a plane of the Hall element, thereby providing a smaller output. To increase a magnetic flux that each magnetic sensor receives, the magnetic sensors A and B (Hall elements) are deviated from the rotating magnet in the rotation axis direction or inclined from the rotation axis of the rotating magnet. To detect the change of the magnetic flux direction, the amplitudes of pluralities of varied outputs should be corrected by an external circuit, resulting in a complicated structure.
To obtain a higher-accuracy output, JP 2003-075108 A discloses a method of averaging outputs from pluralities of magnetic sensors arranged at one position. This method is a signal-treating method adjusting output amplitudes by a circuit, but this reference fails to disclose a method of suppressing the deformation of the output. It also describes that an MR element may be used in place of the Hall element as a magnetic sensor. An MR sensor comprising an MR element (so-called AMR element) is a magnetic sensor for obtaining a 2-period waveform output per one period of an electrical angle, giving two angles at a predetermined output Vo, thus failing to determine an absolute angle.
JP 2000-078809 A discloses a servomotor comprising a rotor having a permanent magnet, and an encoder for detecting the rotation position of the rotor. The permanent magnet has two-pole anisotropy, and the encoder comprises a magnetic sensor for detecting a magnetic field generated from the rotor (leak magnetic flux of a permanent magnet). This reference describes that four magnetic sensors are arranged with an interval of 90° by mechanical angle in a circumferential direction, and a differential signal is obtained between magnetic sensors opposing at 180° to cancel the rotation bias of the rotor, if any, thereby obtaining an absolute position signal with high accuracy. However, the magnetic sensor in the servomotor described in JP 2000-078809 A is not a spin valve type.
JP 2001-343206 A discloses a rotation-angle-detecting apparatus comprising a detection magnet having many magnetic poles on an end surface and a circumferential surface. Specifically, the apparatus comprises a vertical, disc-shaped detection magnet concentric with a rotation shaft, and the detection magnet is concentrically provided on one surface with for instance, 3 pairs (6 poles) of magnetic poles with an equal interval. The detection magnet is also provided on a circumferential surface with, for instance, 48 pairs (96 poles) of magnetic poles with an equal interval. A detection plate is rotatably arranged in parallel with the detection magnet on one surface side thereof with a proper gap. The detection plate is provided with two magnetic detection elements for detecting the magnetic poles at positions along the circumferential surface of the detection magnet, with phase difference of ¼ of the electrical angle defined by 48 pairs of magnetic poles (360°/48/4 by mechanical angle). The detection plate is also provided with three magnetic detection elements for detecting the magnetic poles at positions along one surface of the detection magnet, with phase difference of ⅓ of the electrical angle defined by 3 pairs of magnetic poles (360°/3/3 by mechanical angle). The magnetic detection elements are Hall elements or MR elements. However, the magnetic detection elements in the rotation-angle-detecting apparatus described in JP 2001-343206 A are not of a spin valve type.
JU 62-076607 A (FIG. 1) discloses an apparatus for detecting the rotation angle of a rotating magnet using a ferromagnetic sensor to which a sine-wave signal SW is applied, and a ferromagnetic sensor to which a cosine-wave signal CW is applied. However, the rotation-angle-detecting apparatus of JU 62-076607 A is large and complicated because it needs a signal generator for applying SW and CW to the ferromagnetic sensors. Also, the graph of FIG. 6 indicates that the apparatus comprises a magnetic sensor providing a two-period waveform output per one period of an electrical angle. Because a predetermined output Vo corresponds to two angles, an absolute angle cannot be determined. This ferromagnetic sensor is an MR sensor utilizing a magnetic resistor effect of a ferromagnetic metal. As is clear from the thin-film pattern of FIG. 7, the MR sensor has shape anisotropy in a longitudinal direction. Accordingly, there is unevenness depending on an angle between a magnetization direction and an anisotropy direction when the magnetization rotates, failing to achieve smooth rotation. Thus, an output waveform is deformed, making it difficult to detect the rotation angle accurately.
JP 2002-303536 A (FIGS. 1 and 2) discloses a rotation angle detection sensor comprising sensor substrates opposing each other on the end surface of a two-pole, disc-shaped magnet attached to an end of a rotation shaft. JP 2002-303536 A shows in FIG. 3 that a sensor substrate comprising four GMR elements has a center on a centerline (rotation axis) of the rotation shaft. This GMR element is a spin-valve, giant-magnetoresistive device having a pinned magnetic layer. Because the rotation-angle-detecting sensor of JP 2002-303536 A comprises one sensor substrate having a center on an extension of a center axis of the rotation shaft, the magnet is supported by the rotation shaft in a cantilever manner. Accordingly, this technology cannot be applied to a both-end-support structure having a shaft penetrating a magnet. Also, because the rotation of the magnet is easily deviated, it is difficult to detect the rotation angle accurately. High accuracy of the rotation center (suppression of the variation of magnet rotation) needs a large apparatus.
JP 2006-010346 A (FIG. 4) discloses a magnetic-detection-type position sensor for detecting magnetic flux change caused by the rotation of a magnet attached to a rotating member with a magnetoresistive device to measure the amount of movement of an external movable member. As shown in FIG. 3 of JP 2006-010346 A, however, the deviation of the magnetoresistive device in a direction shown by the arrow L from the center axis of the magnet due to errors in assembling and machining increases the angle error of an output, resulting in difficulty in the accurate detection of the rotation angle.
JP 2006-208025 A (FIGS. 1 and 5) discloses a magnetic sensor comprising a signal magnet whose one rotation corresponds to one period, magnetoresistive devices providing a cosine wave output and a sine wave output with phase difference of a ¼ period, and a bias magnet. In the magnetic sensor described in JP 2006-208025 A, the bias magnet is attached to an MR element to detect the absolute angle. However, the output amplitude of the magnetic sensor varies depending on a ratio of a magnetic field generated by the bias magnet to that generated by the magnet rotor. When the magnet rotor generates a large magnetic field, the output of the magnetic sensor is inverted, resulting in large deformation of the output. Also, when the ratio Bsig/Bbias of the magnetic field (Bsig) of the magnet rotor to the magnetic field (Bbias) of the bias magnet exceeds 0.7, a sinusoidal output cannot easily be obtained, resulting in a deformed output. When the magnet rotor generates a small magnetic field, the magnetic sensor provides a small output. With such tendency, the positional deviation of the magnet rotor in a rotation axis direction provides the output with amplitude variations and deformation, making it difficult to detect the rotation angle with high accuracy.
In addition, the existence of the bias magnet makes it difficult to miniaturize the sensor device. JP 2006-208025 A describes in the paragraph [0029] that the magnetoresistive device is not restricted to a magnetoresistive device (MR element) utilizing an AMR effect, but may be a magnetoresistive device utilizing a GMR effect. However, because the invention described in JP 2006-208025 A is to obtain a one-period output per one rotation of the signal magnet by using the bias magnet, the magnetoresistive device utilizing a GMR effect is not a spin-valve, giant-magnetoresistive device, but a laminated giant-magnetoresistive device, which has a laminate structure with the same function as that of the MR element except for having a larger magnetic resistance change ratio than the MR element. When the bias magnet is used, the laminated giant-magnetoresistive device has the same problem as that of the MR element as described above.
JP 61-142782 A (FIG. 1) discloses a position-detecting apparatus comprising a magnetic recording medium and a ferromagnetic magnetoresistive device opposing the magnetic recording medium, an angle between a pattern surface of the ferromagnetic magnetoresistive device and a longitudinal direction of the pattern being 1-45°. JP 61-142782 A describes that the application of a bias magnetic field to an MR element along its axis of easy magnetization provides a position-detecting apparatus with suppressed instability of a detection output and free from the position shift of an output peak. However, as is clear from FIG. 2 in JP 61-142782 A, a signal representing a resistance change ratio cannot be obtained in a certain angle range near a center between an N pole and an S pole, so that it cannot be used as an absolute angle sensor.
JP 2007-40850 A (FIG. 5) discloses a rotation angle sensor comprising a magnetic sensor arranged in a space above a ring magnet, the magnetic sensor having two magnetoelectric conversion devices for sensing magnetic flux density components Bx and By in parallel with the surface magnet and perpendicular to each other. At the position of the magnetism sensor, the components Bx and By changing with the rotation of the magnet have the same amplitude in an absolute value. FIG. 5 of JP 2007-40850 A indicates that the magnetism sensor detects two axial components Bx and By in parallel with the magnet surface, but it fails to teach the detection of a Z-axis component Bz of the magnetic flux density. Thus, the sensors 12X and 12Y are identified as Hall elements. In order that two sensors are three-dimensionally cross each other as shown in FIG. 5 of JP 2007-40850 A, they should have notches for mating. However, when a magnetosensitive surface of a Hall element as one sensor is at a space position, a notch of the other sensor is at that space position, resulting in deviation. Namely, the rotation angle sensor described in JP 2007-40850 A does not simultaneously detect Bx, By and Bz with one magnetosensitive surface on a substrate. When a sensor is arranged inside a periphery of the magnet, the sensor would come into contact with the magnet if the rotation axis were deviated. On the other hand, when the sensor is arranged outside the magnet with distance, its output decreases drastically because it is a Hall element.
A driving motor mounted on a hybrid vehicle is provided with a resolver for detecting the rotation angle of its rotation axis, to conduct control by changing current for driving the motor and current recovered from the motor. The resolver has a shape similar to the motor, having a yoke having coils wound around a portion on the side of the rotation axis and a portion on the stationary (casing) side. Using the function of the opposing yokes like a transformer, position information about the yoke on the rotation axis side is obtained and converted to the rotation angle. Having the yoke and the coils, however, the resolver is large, heavy and expensive with complicated winding. Accordingly, demand is mounting for small, lightweight sensors capable of detecting the rotation angle with high accuracy, but the above conventional apparatuses are not sufficiently accurate.